This invention relates to a universal key/internal telephone system and more particularly to a line circuit for such a system.
Prior art line circuits are employed where a balanced internal line is required in an internal telephone system. These prior art line circuits employ circuitry that provides DC (direct current) operating current to tip and ring circuits of conventional telephone type subsets. In addition, to DC operating current, these same line circuits provide balanced retard coils for the development of voiced signals over the tip and ring conductor pair. The subsets attached to these line circuits are usually equipped with momentary contact buttons which provide a circuit closure. This circuit closure can be used to activate components in the line circuit that provides the ability to operate an external audio power amplifier for the purposes of paging over a loud speaker system.
The disadvantages of the prior art line circuits is the lack of convenient application to new or existing key telephone systems. The prior art line circuits employ circuit configurations that have power supply polarity and control requirements alien to those found in the conventional key service units (KSU) of a key telephone system. In addition, the physical configuration requires separate housing and wiring facilities. Thus, internal line or lines added to a key telephone system must be housed in their own chassis mounted outside the KSU.